The present invention relates to signal processing in a data storage device and more particularly to a method and system for providing a quasi-current sensing preamplifier having controlled input impedance.
Magnetic recording media, such as disks are widely used for storing data. In order to write to or read from such recording media, storage devices such as disk drives are typically used. Storage devices typically include heads for reading from and/or writing to the recording media. A head is typically attached to a slider, which holds the head above the recording media. The slider is attached to a suspension, which supports the slider above the recording media. The slider is coupled to an actuator arm which moves the head, slider, and suspension across the recording media.
Currently magnetoresistive (MR) heads are increasingly popular for use in read heads. A MR head includes a MR sensor, which has a resistance that varies with an applied magnetic field. The MR sensor may be an anisotropic magnetoresistance (AMR) sensor or a giant magnetoresistance (GMR) sensor. In order to use the MR sensor, the MR sensor is coupled with electronics, which drive the MR sensor and read the signal from the MR sensor. The electronics are coupled with the MR sensor using a conductive interconnect.
The signal from the MR sensor can be read by sensing the current through the MR sensor. In order to do so, a conventional current-sensing preamplifier (conventional CS preamplifier) is used. Typically, an input of the conventional CS preamplifier is coupled with the interconnect and is included in a read/write integrated circuit for the head. Current is input to input devices in the conventional CS via the interconnect, and amplified by a gain stage in the conventional CS. The input devices typically include transistors. The amplified signal is then provided either as a single ended or differential signal to the remainder of the electronics.
Although the combination of the MR sensor, the interconnect, and the conventional CS preamplifier is capable of reading data on recording media, one of ordinary skill in the art will readily recognize that the ability of the storage device to read at higher data rates is limited. Previously, limitations in the bandwidth of the storage device were due to other portions of the electronics. However, improvements in other portions of the electronics have increased the bandwidths of these components of the storage device. As a result, the bandwidth for the storage device has increased. As the data rate increased, the interface between the MR sensor, the interconnect, and the conventional CS preamplifier has become of increasing importance in constraining the bandwidth of the storage device. In particular, the useful bandwidth of the conventional CS preamplifier as used in the storage device is limited. Thus, the ability of the storage device to read at higher data rates is also limited.
Conventional methods for addressing the limited bandwidth of the conventional CS preamplifier also have drawbacks. One reason for the limited bandwidth of the conventional CS preamplifier is the mismatch between the impedance of the MR sensor, a characteristic impedance of the interconnect, and the input impedance of the conventional CS preamplifier. The MR sensor has a resistance which can vary by a factor on the order of two or three to one. The interconnect typically has a relatively low characteristic impedance, generally approximately fifty ohms. The characteristic impedance of the interconnect also changes. Different heads may be used in the storage device. These heads occupy different positions within the storage device. In order to reach these heads, the interconnects may have different lengths or shapes. Furthermore, the interconnect typically twists and bends during use, thereby changing the characteristic impedance of the interconnect. The input devices of the conventional CS preamplifier have a low input impedance, much less than that of the interconnect. Thus, the conventional CS preamplifier has a low input impedance that is much less than the characteristic impedance of the interconnect.
Although conventional voltage-sensing (VS) preamplifiers can have an input impedance that is adjusted, such a solution does not improve the bandwidth of heads utilizing the conventional CS preamplifier. Conventional VS preamplifiers have an input impedance that is very high with respect to the impedance of the interconnect. In conventional VS preamplifiers, the mismatch between the impedance of MR sensor, the characteristic impedance of the interconnect, and the input of the conventional VS preamplifier can be partially addressed by providing a resistor coupled with input devices of the conventional VS preamplifier. The resistor is coupled in parallel with the bases of the transistors used as the input devices. This lowers the input impedance of the conventional VS preamplifier. However, one of ordinary skill in the art will readily recognize that the resistor causes increased signal attenuation, which is undesirable. Furthermore, the input impedance of the conventional CS preamplifier is already lower than the impedance of the interconnect. Consequently, such a method will not improve the bandwidth of the conventional CS preamplifier.
Accordingly, what is needed is a system and method for improving the bandwidth of the conventional CS preamplifier and, therefore, the data rate for the storage device. The present invention addresses such a need.
The present invention provides a method and system for providing a current-sensing preamplifier for use with a magnetoresistive sensor. The method comprises providing at least one input device coupled with the magnetoresistive sensor, providing a gain stage, and providing a feedback circuit. In a system aspect, the current-sensing preamplifier comprises at least one input device coupled with the magnetoresistive sensor. The at least one input device provides an input impedance for the current-sensing preamplifier. The current-sensing preamplifier further comprise providing a gain stage coupled with the at least one input device and a feedback circuit coupled with the gain stage and the at least one input device. The feedback circuit provides at least one signal to the at least one input device. The at least one signal controls the input impedance based on at least one error signal.
According to the system and method disclosed herein, the present invention provides a current-sensing preamplifier having a controllable input impedance. The input impedance of the current-sensing preamplifier can, therefore, be substantially matched to a desired valued. Thus, the operational bandwidth of a system using the current-sensing preamplifier can be increased, allowing for greater ability to read high frequency data from the magnetoresistive sensor.